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Agile Practitioner (practice) is someone who enact the values and principles of Lean and Agile Software Development and enact the practices of many Lean and Agile methods and frameworks, based on what is best for the specific context, project, team, organization and situation.

Agil-ist (ideology)is someone who without distinctions of context or situation take one side and is against the other side, without exceptions. Agil-ist can i.e. take side of Agile against Waterfall or can take side of one favorite Lean-Agile Framework/Method against the other.

Why you shouldn't be an Agil-ist (ideology)?
Every value system, like the Agile's ones, are inevitably incomplete, have some inconsistencies, contradictions and ambiguities:

Idealized value system: a listing of values that lacks exceptions, absolute. Absolutist is indeed someone who hold to idealized value system and claim no exceptions

Realized value system: contains exceptions to resolve contradictions between values in practical circumstances. It is what people tend to use in daily life

Value pluralism: the idea that there are several values which may be equally correct and fundamental, and yet in conflict with each other

Agile is a realized value system that in its roots recognize and deals with inherent ambiguities, contradictions, exceptions, uncertainties and value pluralism that come from fragmented realities, incomplete information, multiple partial views of single individual experiences (read Rigorous Agile from Fowler and The six blind men and the elephant from me) .

Test: are you a Lean and Agile Pratictioner or an Agil-ist? Test yourself : here